The present invention relates to expandable thermoplastic resin particles, and more particularly to expandable particles of an .alpha.-methylstyrene/acrylonitrile polymer having excellent heat resistance, solvent resistance, expandability and moldability and an industrially advantageous process for preparing the expandable particles.
Expandable polystyrene particles are well known expandable thermoplastic particles. Foamed articles can be readily and inexpensively obtained by heating the particles in a mold. However, since the raw material is a polystyrene, the foamed articles are not suitable for use requiring the heat resistance, such as a heat insulating material for pipes of a relatively high temperature, a heat insulating material for roofs, automobile parts or a heat insulating material for solar systems. In particular, in case of using the polystyrene foamed articles as automobile parts in the form of laminates with other materials, selection of adhesives is difficult, because of poor solvent resistance.
It is expected that a copolymer containing at least 10% by weight, especially at least 20% by weight, of .alpha.-methylstyrene and at least 5% by weight of acrylonitrile has heat resistance and solvent resistance sufficient for practical use. Thus, expandable thermoplastic resin particles having good heat and solvent resistances would be obtained by impregnating an expanding agent such as volatile liquid in the resin.
When it is desired to prepare a copolymer containing a large amount of .alpha.-methylstyrene by suspension polymerization to obtain a copolymer having an excellent heat resistance, a process using as a polymerization initiator an organic peroxide such as t-butyl peroxybenzoate, t-butyl peroxyacetate or di-t-butyl peroxide is adoptable. However, in a process using such initiators, it is necessary to use a large amount of the initiators regardless of the polymerization temperature. Consequently, an .alpha.-methylstyrene/acrylonitrile copolymer prepared by such a process is very low in the degree of polymerization, and the expandable particles obtained by impregnating an expanding agent in the copolymer is very poor in foamability. Accordingly, not only satisfactory foamed articles are hard to obtain, but also the foamed articles are low in strength. On the other hand, in case of using these initiators in a decreased amount for the purpose of raising the degree of polymerization, the so-called dead end polymerization occurs, thus a high conversion as required in industrial production is not obtained and, therefore, it is difficult to obtain satisfactory foamed articles. Also, when a general organic peroxide such as benzoyl peroxide is used in the suspension polymerization, the copolymer is not obtained in an industrially satisfactory high conversion even at an optimum temperature for the peroxide.
Like this, the .alpha.-methylstyrene/acrylonitrile copolymers prepared by a conventional suspension polymerization process cannot provide the desired expandable thermoplastic resin particles, or even if obtained, the productivity is very low.
On the other hand, an emulsion polymerization process is known as a process for preparing an .alpha.-methylstyrene/acrylonitrile copolymer having a high degree of polymerization in a high conversion. However, the preparation of expandable particles from the so prepared .alpha.-methylstyrene/acrylonitrile copolymer must be made by coagulating the copolymer from the emulsion, pelletizing the copolymer and then impregnating an expanding agent in the pellets. Not only the process is very complicated, thus resulting in rise of preparation cost, but also the obtained particles are poor in foamability and strength because large amounts of an emulsifier and a coagulating agent are incorporated in the particles.
It is an object of the present invention to provide expandable particles of .alpha.-methylstyrene/acrylonitrile copolymers having excellent heat resistance, solvent resistance, foamability and moldability.
Another object of the invention is to provide a process for preparing expandable particles of .alpha.-methylstyrene/acrylonitrile copolymers having excellent properties in high conversions by a suspension polymerization technique.
These and other objects of the present invention will become apparent from the description hereinafter.